Haplogroup I-M253, also known as I1, is a Y chromosome haplogroup. The genetic markers confirmed as identifying I-M253 are the SNPs M253,M307.2/P203.2, M450/S109, P30, P40, L64, L75, L80, L81, L118, L121/S62, L123, L124/S64, L125/S65, L157.1, L186, and L187. It is a primary branch of Haplogroup I-M170 (I*).

Haplogroup I1 is believed to have been present among Upper Paleolithic European hunter-gatherers as a minor lineage but due to its near-total absence in pre-Neolithic DNA samples it cannot have been very widespread. Neolithic I1 samples are very sparse as well, suggesting a rapid dispersion connected to a founder effect in the Nordic Bronze Age. Today it reaches its peak frequencies in Sweden (52 percent of males in Västra Götaland County) and western Finland (more than 15 percent in Satakunta province). In terms of national averages, I-M253 is found in 38-39% of Swedish males, 37% of Norwegian males, 36.8% of Danish males, 34.5% of Icelandic males, and about 28% of Finnish males. Bryan Sykes, in his 2006 book Blood of the Isles, gives the members – and the notional founding patriarch of I1 the name "Wodan".

All known living members descend from a common ancestor 6 times younger than the common ancestor with I2.

Before a reclassification in 2008, the group was known as I1a, a name that has since been reassigned to a primary branch, haplogroup I-DF29. The other primary branches of I1 (M253) are I1b (S249/Z131) and I1c (Y18119/Z17925).

More than 99% of living men with I1 belong to the DF29 branch which is estimated to have emerged in 2400 BCE. All DF29 men share a common ancestor born between 2500 and 2400 BCE. The oldest ancient individual with I1-DF29 found is Oll009, a man from early Bronze Age Sweden.

A 2024 study found that I1 expanded rapidly during a migration from the eastern or northeastern parts of Scandinavia into Southern Sweden, Denmark and Norway around 2000 BC and was associated with the introduction of stone cist burials. The study concluded that the spread of Y-DNA haplogroup I1 was associated with a genetic cluster labelled as "LNBA phase III" and that this genetic cluster formed the predominant ancestry source for Bronze Age, Iron Age and Viking Age Scandinavians, as well as other ancient European groups with a documented Scandinavian or Germanic association (for example, Anglo-Saxons and Goths; Extended Data Fig. 6e).

While haplogroup I1 most likely diverged from I* as early as 27,000 years ago in the Gravettian, so far DNA studies have only been able to locate it in three Paleolithic and Mesolithic hunter-gatherers. As of November 2022, only 6 ancient DNA samples from human remains dating to earlier than the Nordic Bronze Age have been assigned to haplogroup I1:

Despite the high frequency of haplogroup I1 in present-day Scandinavians, I1 is completely absent among early agriculturalist DNA samples from Neolithic Scandinavia. Except for a single DNA sample (SF11), it is also absent from Mesolithic hunter-gatherers in Scandinavia. I1 first starts to appear in Scandinavia in notable frequency during the late Neolithic in conjunction with the entrance of groups carrying Western Steppe Herder ancestry into Scandinavia, but does not increase significantly in frequency until the beginning of the Nordic Bronze Age.